Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
However, engine run time in hybrid vehicles (HEVs) may be limited, thus limiting engine manifold vacuum, which is typically used to draw fresh air through the fuel vapor canister to desorb the stored fuel vapors. Thus, opportunities for purging fuel vapor from the canister may also be limited. Even if purge conditions are met, the conditions may only be held for a short period of time, leading to incomplete purge cycles. This may result in residual fuel vapors stored in the canister for long periods of time. Typically, the canister is coupled to atmosphere while the vehicle is off. Over the course of a diurnal cycle, the fuel vapors may desorb from the canister as temperature rises, resulting in an increase in bleed emissions. The canister vent valve could be maintained closed, but the vent valve is typically a solenoid valve requiring constant power to stay shut, which could drain the battery if the vehicle is left off for a significant period of time. Further, even when the vehicle is turned back on, the engine may not be used, or may not be operated with an intake vacuum sufficient to purge the vapor canister to intake. Forcing the engine on to evacuate the canister would reduce the efficiency of the hybrid vehicle while increasing combustive emissions.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method for a fuel system, comprising: during a first condition, purging fuel vapor from a fuel vapor canister to a fuel vapor accumulator; and then during a second condition, evacuating fuel vapor from the fuel vapor accumulator to a fuel tank. The fuel vapor accumulator increases the volume of the fuel system, providing a temporary storage for desorbed fuel vapor. In this way, canister breakthrough of hydrocarbons can be mitigated, thereby reducing bleed emissions.
In another example, a fuel system for a vehicle, comprising: a fuel vapor accumulator coupled between a fuel tank isolation valve and a fuel vapor canister; and a reversible vacuum pump coupled between the fuel vapor canister and atmosphere. By incorporating both an accumulator and a reversible vacuum pump in to the fuel system, the system may realize an increased pressure gradient between a sealed fuel tank under vacuum from a diurnal cycle and the rest of the fuel system. In this way, a passive purge operation may return an increased amount of fuel vapor to the fuel tank.
In yet another example, a method for purging a fuel vapor canister, comprising: during a vehicle-off condition, desorbing fuel vapor stored in a fuel vapor canister to a fuel vapor accumulator by pumping air through the fuel vapor canister while maintaining a fuel tank isolation valve and a canister purge valve closed; closing a latchable canister vent valve; responsive to a fuel tank vacuum increasing above a threshold, opening the fuel tank isolation valve and latchable canister vent valve; and then responsive to a fuel tank pressure increasing above a threshold, closing the fuel tank isolation valve. The method enables a canister purge operation while the vehicle is off, leveraging ambient temperature decreases over a diurnal cycle to generate a fuel tank vacuum. In this way, a hybrid vehicle may purge a fuel vapor canister without forcing the engine on, thereby increasing overall fuel efficiency.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.